Plutonic and Hypabyssal Rocks of the

Hailey 1°×2° Quadrangle, Idaho

Prepared in cooperation with the Idaho Geological Survey,

Idaho State University, and the University of Idaho

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Bulletin 2064-U
Plutonic and Hypabyssal Rocks of the

Hailey 1°×2° Quadrangle, Idaho

By Thor H. Kiilsgaard, Reed S. Lewis, and Earl H. Bennett

U.S. Geological Survey Bulletin 2064-U

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Idaho Geological Survey,

Idaho State University, and the University of Idaho
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

This publication is only available online at:

http://geology.cr.usgs.gov/pub/bulletins/b2064-u/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Version 1.0 2001

Published in the Central Region, Denver, Colorado

Manuscript approved for publication December 15, 2000

Graphics by authors and Gayle M. Dumonceaux

Photocomposition by Gayle M. Dumonceaux
III

Contents

Introduction .................................................................................................................................. 1

Cretaceous Plutonic Rocks ........................................................................................................ 1

Quartz Diorite of the Croesus Stock............................................................................. 1

Potassium-Rich Hornblende-Biotite Granodiorite .................................................... 2

Tonalite ............................................................................................................................ 4

Biotite Granodiorite......................................................................................................... 4

Altered Biotite Granodiorite ......................................................................................... 4

Biotite Granodiorite-Aplite and Pegmatite Complexes ............................................ 4

Muscovite-Biotite (Two-Mica) Granite ....................................................................... 5

Leucocratic Granite ....................................................................................................... 5

Pegmatite and Aplite Dikes .......................................................................................... 5

Geochronology ................................................................................................................ 6

Geochemistry .................................................................................................................. 6

Eocene Plutonic Rocks ............................................................................................................... 7

Dioritic Suite..................................................................................................................... 11

Gabbro ....................................................................................................................... 11

Diorite ........................................................................................................................ 11

Foliated Hornblende-Biotite Granodiorite ........................................................... 11

Quartz Monzodiorite ................................................................................................ 11

Biotite Granite.................................................................................................................. 12

Petrographic Differences Between Cretaceous and Eocene

Plutonic Rocks........................................................................................................... 12

Geochemistry of Eocene Plutonic Rocks .................................................................... 12

Hypabyssal Rocks......................................................................................................................... 13

Hypabyssal Rocks of Probable Eocene Age............................................................... 13

Dacitic Suite .............................................................................................................. 14

Dacite .................................................................................................................. 14

Quartz Monzodiorite ......................................................................................... 14

Andesite .............................................................................................................. 14

Rhyolitic Suite............................................................................................................ 15

Rhyolite ............................................................................................................... 15

Rhyodacite .......................................................................................................... 15

Quartz Trachyte ................................................................................................. 15

Basaltic Dikes of Miocene and Younger(?) Age ........................................................ 15

Summary ........................................................................................................................................ 15

References .................................................................................................................................... 16
IV

1. Index map of central Idaho showing location of the Hailey 1°×2° quadrangle,

south-central Idaho, with respect to the Atlanta lobe of the Idaho Batholith ......... 1

2. Simplified geologic map of the Hailey 1°×2° quadrangle showing location 

of Cretaceous and Eocene plutonic rocks and major faults ....................................... 2

3. Modal analyses for samples of Cretaceous and Eocene plutonic rocks, 

Hailey 1°×2° quadrangle ................................................................................................... 3

4. Simplified geologic map of the Hailey 1°×2° quadrangle showing K/Ar 

dates (in Ma) determined during the course of the current study

and those determined previously ..................................................................................... 7

5. Simplified geologic map of the Hailey 1°×2° quadrangle showing Cretaceous 

plutonic rocks and ranges in major-element oxide percentages .............................. 9

Metric Conversion Factors

Multiply By To obtain

Miles 1.609 Kilometers

Feet 0.3048 Meters

Inches 2.54 Centimeters

Tons 1.016 Metric tons

Short tons 0.907 Metric tons

Troy ounces 31.103 Grams

Ounces 28.35 Grams

Figures

Tables

1. Potassium-argon ages of Cretaceous and Eocene plutonic and hypabyssal

rocks, determined from 1979 to 1992, Hailey 1°×2° quadrangle, Idaho .................... 6

2. Mean values of major-element oxides and selected trace elements, 

Cretaceous plutonic rocks, Hailey 1°×2° quadrangle, Idaho ...................................... 8

3. Mean values of major-element oxides and selected trace elements, 

Tertiary plutonic and hypabyssal rocks, Hailey 1°×2° quadrangle, Idaho ................ 16
1

0 50 KILOMETERS 25

Boise

Hailey

Area of Hailey 

1° x 2° quadrangle

ATLANTA LOBE

45°

44°

43°

116° 114°

Plutonic and Hypabyssal Rocks of the 

Hailey 1°×2° Quadrangle, Idaho

By Thor H. Kiilsgaard, Reed S. Lewis, and Earl H. Bennett

Introduction

Much of the area of the Hailey 1°×2° quadrangle, Idaho, is 

underlain by Cretaceous plutonic rocks of the Atlanta lobe of the 

Idaho batholith (fig. l). The oldest phases of the Atlanta lobe 

consist of hornblende-bearing tonalite and granodiorite and 

minor pyroxene diorite. Younger phases include voluminous 

amounts of biotite granodiorite and lesser amounts of two-mica 

granite and leuococratic granite. Intrusive into these rocks are 

Eocene stocks and batholiths that range in composition from 

gabbro to younger granite. Cutting all of the plutonic rocks are 

swarms of Tertiary hypabyssal dikes that range in composition 

from rhyolite to andesite and include.

 In the following rock descriptions, rock unit symbols refer 

to rock units of the Hailey 1°×2° geologic map by Worl and others (1991). A simplified version of that map, using different 

symbols, is shown in Worl and others (1991). Most geographic 

place names in the various descriptions are shown on the Hailey 

1°×2° geologic map, and all place names are shown on the 

larger scale 7.5-minute topographic maps that were used as base 

maps in the geologic mapping of the region.

Cretaceous Plutonic Rocks

Twelve rock units of Cretaceous age are shown on the 

Hailey 1°×2° geologic map (Worl and others, 1991). Figure 2 is 

a simplified geologic map showing the major plutonic phases. 

Most units cover large areas, on the order of 100 km2. The 

biotite granodiorite unit is an immense mass of relatively homogeneous rock and is widely exposed in central Idaho. Individual 

zoned plutons have been mapped as outliers of the southeastern 

part of the batholith, but the main mass of the batholith is only 

crudely zoned. This zonation consists of a tonalitic western 

margin (west of the Hailey 1°×2° quadrangle) that is intruded in 

the more central part of the Atlanta lobe by biotite granodiorite 

and two-mica granite. In the southeastern part of the lobe are 

more mafic rocks that probably were emplaced at about the 

same time as the tonalitic phase on the west side. 

Modes of representative samples from Cretaceous rock 

units are shown on figure 3. The various lithologies of Cretaceous plutonic rocks are described in the following sections.

Quartz Diorite of the Croesus Stock

The Croesus stock (fig. 2, unit kqd) is a northwest-trending body of quartz diorite grading to diorite that crops out in the 

hills west of Bellevue in the southeastern quarter of the quadrangle (Worl and others, 1991). At the Minnie Moore mine, the 

stock has an intrusive, sill-like relationship with the overlying 

Milligen Formation of Devonian age. Previous descriptions of 

the stock include those of Lindgren (1900), Hewett (1930), and 

Anderson and others (1950). Although separated from the main 

mass of the Atlanta lobe to the west, the Croesus stock as well 

as the Deer Creek and Rooks Creek stocks (fig. 2) (which differ 

in composition from the Croesus stock) are considered to be 

units of the Atlanta lobe.

The quartz diorite is dark gray, medium grained and 

equigranular and is characterized by subequal amounts of augite 

and vitreous biotite that together compose about 20 percent of 

the rock. Hypersthene is present in minor amounts, as is hornblende, which has replaced pyroxene. Andesine, the principle 

Figure 1. Index map of central Idaho showing location of the Hailey 

1°×2° quadrangle, south-central Idaho, with respect to the Atlanta lobe 

of the Idaho batholith. Cretaceous plutonic rocks exposed in the region 

are light shaded; Eocene plutonic rocks are dark shaded.
2 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

EXPLANATION

Biotite granite

Dioritic suite

Muscovite-biotite granite

Biotite granodiorite (includes

 pegmatite-rich areas)

Hornblende-biotite tonalite

Potassium-rich hornblende-biotite

 granodiorite

Quartz diorite

Fault

Td

Kgd

Tg

Kg

Kt

Kgdk

Kqd

0

0 10

10 20 30 KILOMETERS

20 MILES

Idaho City

Atlanta

Fairfield

Hailey

Mt

Heinen

Trinity

Mtn

House

Mtn

Smoky

Dome

Buttercup Mtn

Steel

Mtn

Swanholm

Peak

Sheep Creek

batholith

Sawtooth

batholith

Rooks Creek

stock

Deer Creek stock

Croesus

stock

Mountain Home

44°

43°30'

43°

116° 115° 114°

constituent, is euhedral, commonly aligned, and slightly zoned. 

Quartz is interstitial and composes less than 15 percent of the 

rock; potassium feldspar is present in sparse amounts. Accessory 

minerals include sphene, apatite, magnetite, and ilmenite. 

Modal analyses are shown in figure 3D.

A unique feature of the stock is the abundance of silverlead-zinc deposits that are peripheral to or within it (Hewett, 

1930; Anderson and others, 1950). In this respect, the deposits appear to be closely associated with the quartz diorite of 

the stock.

The age of the Croesus stock has not been determined. 

Sericite from an alteration selvage at the Magdalena mine, northwest of Bellevue, yielded a 40Ar/39Ar plateau age of 85.6+0.6 

Ma (L.W. Snee, unpub. data, 1992). The west side of the stock is 

intruded by hornblende-biotite granodiorite from which a 

sample, taken at the McCoy mine, yielded a K-Ar age on biotite 

of 83.8+2.5 Ma (recalculated from Berry and others, 1976). The 

quartz diorite, consequently, must be older than about 86 Ma.

Potassium-Rich Hornblende-Biotite Granodiorite

Hornblende-biotite granodiorite is widespread in the southeastern quarter of the quadrangle in the area northeast of Fairfield and southeast of Buttercup Mountain (Worl and others, 

1991) (fig. 2, unit Kgdk). The unit grades from granodiorite to 

granite (fig. 3D) and is medium to dark gray, equigranular to 

porphyritic, and medium to coarse grained. It has a higher K2O 

content for a given SiO2 content than other Cretaceous units of 

the batholith (Lewis, 1989, 1990). The rock is characterized by 

Figure 2. Simplified geologic map showing location of Cretaceous and Eocene plutonic rocks and associated major faults in the Hailey 1°×2°

quadrangle.
3

A

A P

Q

GRANITE

GRANODIORITE

TONALITE

ALKALI-FELDSPAR GRANITE

QUARTZ

SYENITE

QUARTZ

MONZONITE

QUARTZ

MONZODIORITE

QUARTZ

DIORITE

SYENITE MONZONITE MONZODIORITE

DIORITE/

GABBRO

60

20

5

90 65 35 10

B

A P

Q

C

A P

Q

D

A P

Q

E

A P

Q

F

A P

Q

coarse books (as thick as 6 mm) of vitreous biotite that contrast 

with the small flakes of biotite in the widespread biotite granodiorite (unit Kgd). Subhedral crystals of hornblende range from 

1 to 7 mm in length. The principal constituent is plagioclase, 

which ranges from oligoclase to andesine in composition and 

exhibits normal to oscillatory zoning. Quartz in the rock commonly forms polycrystalline aggregates as much as 10 mm 

across that locally contain needles of rutile. The potassium 

feldspar is pale pink microcline that displays poor to strongly 

developed grid twins and weak microperthitic texture. Accessory minerals include sphene and apatite and lesser quantities of 

allanite and zircon.

The western contact of potassium-rich hornblende-biotite 

granodiorite with the more extensive biotite granodiorite is not 

well exposed, and the age relationships of the two units were not 

determined by geologic mapping. Along the eastern contact, in 

the vicinity of the McCoy mine, the hornblende-biotite granodiorite intrudes the older quartz diorite.

Figure 3. Modal analyses for samples of Cretaceous and Eocene plutonic rocks, Hailey 1°×2° quadrangle. Analyses normalized to 100 percent 

quartz (Q), alkali feldspar (A), and plagioclase (P). Closed circles indicate modal values; open circles indicate mean values. A, rock classification 

from Streckeisen (1973); B, Cretaceous biotite granodiorite (fig. 2, unit Kgd); C, muscovite-biotite (two-mica) granodiorite and granite (fig. 2, unit 

Kg); D, potassium-rich, hornblende-biotite granodiorite (fig. 2, unit Kgdk) and quartz diorite (fig. 2, unit Kqd); E, Eocene biotite granite (pink granite)(fig. 2, unit Tg); and F, dioritic suite (fig. 2, unit Td).

Cretaceous Plutonic Rocks
4 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

Two K/Ar dates on biotite of 84.0 and 83.8 Ma (Lewis and 

others, 1987; Berry and others, 1976) are the best estimate of the 

age of the hornblende-biotite granodiorite unit.

Tonalite

Hornblende-biotite tonalite grading to granodiorite crops 

out along the western edge of the Challis 1°×2° quadrangle 

(Kiilsgaard and Lewis, 1985). Outcrops of similar rock are 

present west of the Hailey 1°×2° quadrangle but not in exposures 

of sufficient size to warrant mapping as individual units. In the 

central part of the quadrangle, tonalite grading to granodiorite 

was mapped along the western side of House Mountain and 

along both sides of the South Fork of the Boise River, from near 

the mouth of Beaver Creek east to near the junction of Big 

Smoky Creek and the South Fork of the Boise River (Worl and 

others, 1991) (fig. 2, unit Kt).

The tonalite is medium gray, equigranular, and medium 

grained. Biotite locally defines a weak foliation, and biotite-rich 

schlieren are present at a few localities. The unit is crosscut by 

numerous aplite and pegmatite dikes and, to a much lesser 

extent, by fine-grained granitic dikes that may be offshoots of 

the biotite granodiorite unit discussed in the following section. 

Although this dike relationship suggests that the tonalite predates the biotite granodiorite, several of the contact areas 

between the two units appear to be gradational, and the two units 

may be similar in age.

Plagioclase feldspar in the tonalite is normally zoned 

andesine and calcic oligoclase. Potassium feldspar lacks albitepericline grid twins, and microperthitic textures are weakly 

developed or absent. Quartz is present as aggregates of small 

(0.5 mm) grains formed by recrystallization of larger strained 

grains. Subhedral biotite make up about 10 percent of the rock. 

Lesser amounts of green hornblende are present as stubby, subhedral crystals as long as 6 mm. Magnetite is the predominant 

opaque mineral. Apatite and sphene are conspicuous accessory 

minerals, and lesser amounts of allanite and zircon are also 

present.

An 40Ar/39Ar plateau age of 93.7+0.3 Ma on hornblende 

was obtained from tonalite along the South Fork of the Boise 

River (L.W. Snee, unpub. data, 1992).

Biotite Granodiorite

Biotite granodiorite is widespread over a vast area in central 

Idaho. It extends more than 130 km in an east-west dimension 

and more than 190 km in a north-south dimension (Fisher and 

others, 1983; Worl and others, 1991). Biotite granodiorite 

makes up most of the Idaho batholith in the Hailey 1°×2° quadrangle (fig. 2, unit Kgd) and the Challis 1°×2° quadrangle to the 

north. It is the principal granitic rock exposed along the various 

roads in that part of Idaho, such as north along State Highway 21 

from the mouth of Grimes Creek, past Idaho City, to and beyond 

Lowman. 

The biotite granodiorite is light gray, medium to coarse 

grained, and equigranular to porphyritic. Phenocrysts, where 

present, are potassium feldspar and are ±2 cm. Plagioclase 

(weakly zoned oligoclase) is the principal mineral in the rock, 

and quartz and potassium feldspar are also present (fig. 3B). 

Quartz commonly is present as large clots that are somewhat 

darkened and much like phenocrysts. Under the microscope, 

the clots are seen to be contiguous aggregates of crystals and not 

large single crystals. Biotite makes up 2-12 percent of the rock 

and commonly is partly altered to chlorite. Sericite, an alteration product of feldspar, is widespread throughout the rock, but 

primary muscovite is sparse to absent. Hornblende is not 

present in biotite granodiorite in the western part of the quadrangle but is locally present east of the Middle and South Forks 

of the Boise River. Accessory minerals include sphene, allanite, 

zircon, monazite, and opaque minerals.

The age of the biotite granodiorite is not well established, 

but the unit probably was intruded between about 85 and 70 Ma.

Altered Biotite Granodiorite

Altered biotite granodiorite (unit Kai of Worl and others, 1991) crops out at several localities in the western part of 

the quadrangle. It is conspicuous on the high ridge at the 

headwaters of California and Grayback Gulches, about 5 km 

south of Idaho City, and extending along the ridge to the 

southwest. Other exposures are on both sides of Mores Creek 

about 8 km southwest of Idaho City and west, south, and east 

of Mt. Heinen.

Altered biotite granodiorite is light gray, although commonly it is iron stained along or near fracture surfaces. The 

rock is medium- to coarse grained and equigranular to porphyritic. Feldspar in the rock is moderately to intensely altered to 

sericite and clay minerals. Commonly, feldspar phenocrysts are 

so altered that identification of the original mineral is uncertain. 

Biotite is altered to chlorite or to iron oxide minerals, which 

accounts for the reddish, rusty stain that is characteristic of the 

unit. Locally, the rock is moderately to intensely silicified, 

either by silica flooding of the matrix, or by countless veinlets of 

quartz, that commonly are 2 mm thick or less.

All mapped exposures of altered biotite granodiorite are 

near younger plutonic rocks of Eocene age that clearly intrude 

the older altered rocks. Field relationships and alteration features of the older rocks indicate that the alteration is the result of 

hydrothermal activity associated with the younger intrusive 

activity.

Biotite Granodiorite-Aplite and Pegmatite 

Complexes

Near the southern edge of the Idaho batholith, in the area 

between House Mountain and Smoky Dome, large exposures of 

biotite granodiorite are cut by so many aplite and pegmatite 

dikes (the dikes make up more than 50 percent of the exposed 

mass) that the rocks are identified as a separate unit (unit Kap of 

Worl and others, 1991). Locally, the dikes, which are considered to be Cretaceous in age, are so dominant that they give the 

host biotite granodiorite the appearance of being engulfed, 
5

ellipsoidal-shaped xenoliths (Bennett, in press). Northwest of 

Hill City, the biotite granodiorite-dike complex is foliated (unit 

Kfap of Worl and others, 1991), the biotite in the granodiorite 

marking the foliation. In the area around Anderson Ranch Reservoir, the granodiorite-dike complex contains pendants and 

xenoliths of schist, quartzite, and various calc-silicate rocks in 

such abundance that the rocks are identified as a mappable unit 

(unit Kapm of Worl and others, 1991).

The three biotite granodiorite-dike complex units mark the 

top or near top of the Idaho batholith. This conclusion is based 

on observations at roof pendants, both in the Hailey 1°×2° quadrangle and in the contiguous Challis 1°×2° quadrangle to the 

north (Kiilsgaard and Lewis, 1985).

Muscovite-Biotite (Two-Mica) Granite

A southwest-trending belt of muscovite-biotite (two-mica) 

granite grading to granodiorite extends across the northwestern 

corner of the quadrangle (fig. 2, unit Kg) (Worl and others, 

1991). The rock also is exposed from the vicinity of Sunset 

Mountain southeast to the vicinity of Swanholm Creek and 

along the southern edge of the Idaho batholith in the areas north 

of Mayfield and west of Smith Prairie.

Laminated plates or books of primary muscovite, large 

enough to recognize in hand sample, were used as a field criterion in mapping muscovite-biotite granite. The rock is massive, 

light gray, medium to coarse grained, and equigranular to porphyritic and consists chiefly of oligoclase, quartz, and potassium feldspar. Muscovite makes up as much as 5 percent of the 

rock but averages about 2 percent. Biotite content averages less 

than 5 percent. Hornblende is not present in the two-mica granite. Red garnet is a common accessory. The average mode of 

samples of the rock from the Hailey 1°×2° quadrangle plots as 

granodiorite (fig. 3C), but in the Challis 1°×2° quadrangle to the 

north the average mode plotted as granite (Kiilsgaard and 

Lewis, 1985).

Except at fault contacts, the contact between muscovitebiotite granite and biotite granodiorite is gradational across 

widths of as much as 3 km or more. A K/Ar age determination 

of 67.2+2.5 Ma on biotite is interpreted as a minimum age of the 

two-mica unit. Similar muscovite-bearing granite in the Challis 

1°×2° quadrangle has muscovite K/Ar ages of 76-69 Ma (Lewis 

and others, 1987).

Leucocratic Granite

Dikes and small stocks of leucocratic granite as large as 5 

km in maximum dimension crop out in the north-central part of 

the quadrangle, northwest of Atlanta, in the vicinity of Johnson 

Creek and the North Fork of the Boise River (Kiilsgaard, 

1983a) (unit Klg of Worl and others, 1991). Dikes of leucocratic granite are present, but less common, in the western part 

of the quadrangle.

The leucocratic granite is light gray to almost white and 

fine to medium grained and has a distinctive aplitic texture. The 

rock consists almost entirely of quartz, plagioclase, and 

potassium feldspar. Accessory minerals include biotite, magnetite, sphene, and allanite. Garnets as much as 3 mm across are 

common at some exposures. The plagioclase (An 26-30) is 

moderately altered to sericite, as is the potassium feldspar, 

which is chiefly perthitic microcline. Seventeen samples of the 

rock collected in the north-central part of the quadrangle and the 

area immediately to the north of the quadrangle have an average 

mode of 34 percent quartz, 32 percent potassium feldspar, and 

34 percent plagioclase. Mafic mineral content, chiefly biotite, 

ranges from less than 1 percent to 3 percent but averages about 2 

percent.

Distinguishing features of the rock are its fine to medium 

grain size and light color. The rock is resistant to erosion and 

tends to form high points of ridges. It weathers to irregular, 

resistant, rubbly scree that, on steep hillsides, gives the appearance of having been eroded from larger bodies, when, in fact, 

most rubbly scree is derived from dikes that are only about 2–4 

m thick.

The leucocratic granite cuts sharply across all other plutonic rocks of the Idaho batholith. On the high ridges northeast 

of Goat Mountain it is intruded by Eocene biotite (pink) granite, 

and in upper reaches of Little Silver Creek, immediately to the 

north of the Hailey 1°×2° quadrangle, it is intruded by Eocene 

porphyritic biotite-hornblende granodiorite. K/Ar age determinations from leucocratic granite in the Challis 1°×2° quadrangle 

suggest an age of about 75–70 Ma (Lewis and others, 1987).

Pegmatite and Aplite Dikes

Pegmatite and aplite dikes commonly crosscut Cretaceous 

plutonic rocks in the Hailey 1°×2° quadrangle, but most of the 

dikes are small, only a few centimeters thick and a few meters 

long, and were not mapped. Locally, pegmatite dikes are 2-3 m 

thick but only ±10 m long. The larger pegmatite dikes are 

shown on the geologic map of Worl and others (1991). The 

dikes are most common along the crests of high ridges, and their 

resistance to erosion probably is a factor in the resistance of the 

ridges to erosion. In the south-central part of the quadrangle the 

dikes are so numerous that they form a complex of rocks 

described earlier as the aplite-pegmatite complex.

Aplite dikes rarely are more than 5-15 cm thick. The aplite 

has aphanitic texture and is composed chiefly of quartz and feldspar. The pegmatite is light, almost white in color, and is distinguished by the large grain size of the minerals. Sheets of 

muscovite, for example, commonly are as much as 15 cm in the 

maximum dimension. Quartz may be graphically intergrown 

with feldspar. The largest pegmatite dike crops out immediately 

north of the northern boundary of the Hailey 1°×2° quadrangle, 

near its northwestern corner. That dike, one of three in the area, 

is about 75 m long and 60 m wide. It has a graphic granite wall 

zone, a plagioclase-quartz-muscovite intermediate zone, and a 

quartz core. Columbite, samarskite, and monazite are in the 

intermediate zone. A 140 kg crystal that has a core of columbite 

and a rim of samarskite was reportedly found in the pegmatite 

(Fryklund, 1951). Also in the northwestern corner of the quadrangle, about 1.2 km upstream from the mouth of Swede Creek 

and at the stream bank, is a pegmatite about 2 m thick and 12 m 

Cretaceous Plutonic Rocks
6 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

long from which a few kilograms of colorless beryl and some 

topaz reportedly was mined in 1987.

Age determinations of the pegmatite and aplite dikes were 

not attempted, but field relationships indicate that they are associated with the Cretaceous plutonic rocks.

Geochronology

Although various age designations have been proposed for 

the Cretaceous plutonic rock units in the Hailey 1°×2° quadrangle, the ages of plutonic and hypabyssal rocks in the quadrangle 

are not well known. Most radiometric determinations were 

made using the K/Ar method, which records only the time since 

the rock cooled past the argon blocking temperature of the mineral dated. Also, in some areas, the Cretaceous plutonic rocks 

were reheated by Eocene plutonism, resulting in argon loss from 

the biotite and muscovite. A number of samples were collected 

along or near major faults, and hydrothermal activity along the 

faults also has caused argon loss. In some cases hornblende may 

have gained argon during reheating and the dates tend to be too 

old. Recent dating by the 40Ar/39Ar method (L.W. Snee, unpub. 

data, 1992) helps to define the effects of argon loss and gain and 

to obtain more reliable dates. Overall, however, age data for 

rocks of the Atlanta lobe of the batholith are sparse, and the K/

Ar dates contain many uncertainties.

Age determinations of Cretaceous and Eocene rocks by 

the K/Ar method made during the course of the present study 

are presented in table 1. Field localities of known dated samples are shown in figure 4. Because of reasons discussed preceding, most of the age determinations of Cretaceous samples 

are considered to be too young. Nevertheless, a review of 

available data indicates that Cretaceous intrusive activity began 

about 95 Ma and continued until about 70-65 Ma (table 1) 

(Armstrong, 1975; Berry and others, 1976; Criss and others, 

1982; Lewis and others, 1987; Lewis, 1990; Park, 1990; L.W. 

Snee, unpub. data, 1992).

Geochemistry

Mean values of major-element oxides and some trace elements for 90 samples comprising seven rock types are shown in 

table 2. Average SiO2 content ranges from 57 to 75 percent, but 

the most common rock types contain between 66 and 74 percent. Average biotite granodiorite in the western part of the 

quadrangle differs in chemical content from that of petrographically similar biotite granodiorite in the eastern part of the quadrangle. The western biotite granodiorite is more enriched in 

SiO2, Na2O, and Ba but has a lower average content of iron 

oxides, MgO, CaO, TiO2, P2O5, Y, and Zr. These chemical differences probably result from different magmatic source regions 

from west to east. The only field distinction between western 

and eastern biotite granodiorite is the local presence of hornblende in the eastern granodiorite. Chemically, granodiorite in 

the east contains less than about 70.5 percent SiO2. The chemical and mineralogical changes occur across a poorly constrained 

northerly trending zone that passes through the vicinity of 

Atlanta (fig. 5) and appears to be gradational. East of this zone, 

biotite granodiorite contains less SiO2 and Na2O and more of 

MgO and CaO (fig. 5). Hornblende-biotite granodiorite, 

tonalite, and quartz diorite also contain less SiO2 than does 

western granodiorite. Chemical differences are less apparent 

between eastern granodiorite and potassium-enriched hornblende-biotite granodiorite because only average values are 

reported. For a given SiO2 content eastern granodiorite is 

depleted in K2O and MgO and enriched in Na2O, Ba, and Sr relative to hornblende-biotite granodiorite (Lewis, 1989, 1990).

Muscovite-biotite granite is similar chemically (table 2) 

and, except for muscovite, similar mineralogically to western 

Table 1. Potassium-argon ages of Cretaceous and Eocene plutonic and hypabyssal rocks, determined from l979 to 1992, Hailey 1°×2° 

quadrangle, Idaho.

[Dated by R.F. Marvin and Jarel Van Essen, U.S. Geological Survey, except for samples IC-55 and IC-77, which were dated by Krueger Enterprises, 

Inc., Geochron Laboratories Division and the data donated by Lawrence F. Baum, of L.F. Baum and Associates. Rock units: Kgd, biotite granodiorite; 

Kg, muscovite-biotite granite; Tfgd, foliated hornblende-biotite granodiorite; Tqm, quartz monzodiorite; Tg, biotite granite; Tda, dacite. Listed 40Ar 

radiometric (rad) values are 10-10 mole/gram]

Sample Latitude Longitude Rock Mineral K2O 40Ar 40Ar Age (Ma)

(degrees (degrees unit percent radio- percent

 north) west) metric

B8828501 43.59306 115.01000 Kgd Hornblende 1.34 1.901 75 96.0 + 3.5

RL-193 43.70000 115.99917 Kgd Hornblende 1.29 1.980 85 104.0 + 4.0

85-51K 43.90653 115.64500 Kg Muscovite 10.20 10.06 80 67.2 + 1.9

85-51K 43.90653 115.64500 Kg Biotite 8.58 8.063 90 64.1 + 2.3

84-34K 43.77597 115.92181 Tqm Biotite 6.91 4.640 86 46.1 + 1.7

85-301K 43.71000 115.80167 Tqm Biotite 7.46 4.764 73 43.8 + 1.6

85DS61 43.74167 114.72500 Tqm Biotite 4.57 3.214 77 48.2 + 1.7

IC-55 43.95235 115.99054 Kgd Biotite 8.545 8.453 83 67.2 + 2.5

IC-58 43.98057 115.90665 Tqm Biotite 7.480 4.973 80 45.5 + 1.8

P-87 43.55000 114.80000 Tfgd Biotite 8.73 5.526 65 43.5 + 1.3

P-147 43.60000 115.95000 Tqm Biotite 8.73 5.271 76 41.5 + 1.2

P-150-1 43.65000 115.78330 Tg Biotite 8.85 5.309 51 41.2 + 1.2

H-173 43.60500 114.7083 Tda Hornblende 0.845 0.5849 42 47.2 + 0.9
7

EXPLANATION

Biotite granite

Dioritic suite

Muscovite-biotite granite

Biotite granodiorite (includes

 pegmatite-rich areas)

Hornblende-biotite tonalite

Potassium-rich hornblende-biotite

 granodiorite

Quartz diorite

Fault

Td

Kgd

Tg

Kg

Kt

Kgdk

Kqd

0

0 10

10 20 30 KILOMETERS

20 MILES

Idaho

City Atlanta

Fairfield

Hailey

Mountain Home

Mt

Heinen

Trinity

Mtn

House

Mtn

Smoky

Dome

Buttercup

Mtn

Steel Mtn

Swanholm

Peak

Sheep Creek

batholith

Sawtooth

batholith

Rooks Creek

stock

Deer Creek stock

Croesus

stock

45.5

67.2

60.4

67.2 (mu)

64.1 (bt)

65.6

60.1 44.6

37.2

43.8

46.1

57.5

38.7

41.5 45.0

41.2 44.2

40.5

53.9

53.4

44.1

63.2

45.3

44.8

45.6

48.2

104

96.0 46.1 43.5

76.4

84.0

83.8

92.2

76.6 (bt)

114 (hb)

38.0

44°

43°30'

43°

116° 115° 114°

biotite granodiorite, as is leucocratic granite. Western biotite 

granodiorite, muscovite-biotite granite, and leucocratic granite 

are peraluminous (table 2, A/CNK ratios), whereas hornblendebearing rocks (eastern biotite granodiorite, hornblende-biotite 

granodiorite, hornblende-biotite tonalite, and quartz diorite) that 

are east of the northerly trending zone near Atlanta are metaluminous to weakly peraluminous.

Eocene Plutonic Rocks

Large areas of the Hailey quadrangle previously thought to 

be underlain by Cretaceous plutonic rocks of the Idaho batholith 

are now known to be underlain by Eocene plutonic rocks (fig. 2) 

(Worl and others, 1991). Two categories of Eocene plutonic 

rocks are recognized: a dioritic suite that ranges from gabbro to 

granite in composition but is chiefly quartz monzodiorite, and 

biotite granite (commonly called pink granite). The designation 

“dioritic suite” is used herein as a means of showing the various 

intermediate to more mafic Eocene rocks as a single unit in figure 2 (unit Td). Rocks of the two categories are present as discrete stocks and epizonal batholiths that intrude the Cretaceous 

Idaho batholith as well as Paleozoic metasedimentary rocks in 

the eastern part of the quadrangle (Bennett, 1980a, b; Bennett 

and Knowles, 1985; Lewis and Kiilsgaard, 1991). Intrusive contacts with pre-Tertiary rocks commonly are sharp, and contact 

metamorphic effects are minimal. Rocks of the two categories 

commonly are in sharp contact with each other and have been 

intruded along similar northeast-trending structures (Fisher and 

others, 1983; Worl and others, 1991). At two localities, apophyses of biotite granite cut the dioritic rocks, and, elsewhere, 

Figure 4. Simplified geologic map of Cretaceous and Eocene plutonic rocks and associated major faults in the Hailey 1°×2° quadrangle, showing 

K/Ar dates (in Ma) determined during the course the current study (solid circles), and those determined previously (triangles) (Criss and others, 

1982). mu, muscovite; bt, biotite; hb, hornblende.

Eocene Plutonic Rocks
8 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

remnants of the dioritic rocks are present in the biotite granite. 

These field observations suggest that the dioritic rocks are older, 

a conclusion supported by age determinations of the rocks in the 

contiguous Challis 1°×2° quadrangle to the north (Bennett and 

Knowles, 1985).

New potassium-argon age determinations of Eocene plutonic rocks are shown in table 1. A summary of other age 

determinations is in Criss and others (1982). Locations of all 

known dated Eocene rocks for which sample determinations 

are considered reliable are shown in figure 4. In contrast to 

the problems associated with K/Ar dating of Cretaceous rocks, 

K/Ar age determinations of Eocene plutonic rocks are probably reasonably accurate. Most ages are between 48 and 40 Ma 

and are consistent with field relationships, which is probably a 

result of the epizonal emplacement levels and relatively rapid 

cooling rates of the Eocene rocks. Also, the area has not 

undergone major heating since Eocene time. Of the reported 

determinations, 11 samples of dioritic rocks show an age 

range from 48.4 Ma to 38.7 Ma and an average of 44.9 Ma, 

whereas 7 samples of biotite (pink) granite show an age range 

from 45.6 Ma to 37.2 Ma and an average of 43.0 Ma. The 

determinations indicate that, on average, the dioritic rocks are 

slightly older than the biotite granite, a relationship supported 

by field observations.

The close spatial relationship between the biotite granite 

and the dioritic suite was noted in the contiguous Challis 1°×2°

quadrangle to the north (Fisher and others, 1983; Kiilsgaard, 

1983a; Bennett and Knowles, 1985). In the Hailey 1°×2° quadrangle, the relationship is particularly evident along the northwest and southeast sides of the Sheep Creek batholith and along 

the periphery of much of the Smoky Dome stock. Although the 

close spatal relationship suggests a genetic link between the two 

types of rocks, isotopic and major- and trace-element geochemical data indicate that the two categories of rocks are not related 

through fractional crystallization (Lewis, 1984; Stewart, 1987; 

Lewis and Kiilsgaard, 1991; Stewart and others, in press). 

Rather, the respective magmas probably originated from different sources. The magmas could have been related to the same 

thermal event or to closely spaced events, and they could have 

been emplaced along the same zones of structural weakness 

(that is, the Trans-Challis fault system and related structures 

Table 2. Mean values of major oxides and trace elements, Cretaceous plutonic rocks, Hailey 1°×2° quadrangle, Idaho.

[Analytical determinations by X-ray fluorescence and optical spectroscopy. Analysts: T. Taggart A. Bartel, T. McCollom, P. Briggs, E. Brandt, 

C. Papp, C. Stone, S. Roof, L. Jackson, J. Christie, K. Stewart, D. Siems, K. Slaughter, N. Elsheimer, J. Evans, M. Malcolm, and S. Pribble, U.S. 

Geological Survey. Rock units: Kgd, biotite granodiorite; Kgdk, potassium-enriched hornblende-biotite granodiorite; Kt, tonalite; Kgd, quartz 

diorite; Kg, muscovite-biotite granite; and Klg, leucocratic granite. A/CNK, Al2O3/(CaO+Na2O+K2O) molecular ratio]

Rock unit Kgd Kgd Kgdk Kt Kqd Kg Klg

(west) (east)

No. of samples---------------- 31 27 11 4 4 7 6

In Percent

SiO2 72.4 68.9 67.3 65.8 56.8 73.6 75.3

Al2O3 15.1 15.4 15.1 16.1 17.1 14.6 14.1

Fe2O3 0.35 1.19 1.26 1.43 1.61 0.49 0.30

FeO 0.77 1.55 2.10 2.25 4.62 0.52 0.42

MgO 0.28 0.90 1.68 1.49 4.06 0.24 0.17

CaO 1.67 3.07 3.54 3.95 6.99 1.53 0.85

Na2O 4.40 3.64 3.27 4.04 3.47 4.16 4.17

K2O 3.37 3.14 3.43 2.14 1.69 3.04 4.17

TiO2 0.14 0.49 0.56 0.66 1.16 0.11 0.06

P2O5 0.04 0.16 0.18 0.22 0.40 0.05 0.05

MnO 0.03 0.04 0.06 0.06 0.10 0.03 0.02

A/CNK 1.09 1.00 0.98 1.00 0.84 1.13 1.09

In Parts Per Million

Ba 1550 1310 910 1260 880 1460 1180

Sr 520 700 500 790 840 470 340

Y 6 12 13 14 20 9 5

Zr 90 210 170 220 180 80 20

Rb 80 100 120 80 50 80 80

Figure 5 (following pages). Simplified geologic map of the Hailey 

1°×2° quadrangle showing Cretaceous plutonic rocks and ranges in major-element oxide percentages. All analyses by U.S. Geological Survey 

laboratories as part of current study except for eight samples of biotite 

granodiorite from the Cayuse Point area south of Atlanta (Slavik, 1987). 

A, SiO2. B, Na2O. C, MgO. D, CaO.
9

0 10 20 30 KILOMETERS

Idaho City Atlanta

Fairfield

Mountain Home

Hailey

44°

43°30'

43°

116° 115° 114°

EXPLANATION

Muscovite-biotite granite

Biotite granodiorite (includes

 pegmatite-rich areas)

Hornblende-biotite tonalite

Potassium-rich hornblende-

 biotite granodiorite

Quartz diorite

Kgd

Kg

Kt

Kgdk

Kqd

A

SiO2 < 65.5

SiO2 ≤ 65.5 to < 70.5

SiO2 ≥ 70.5

Idaho City Atlanta

Fairfield

Hailey

0 10 20 30 KILOMETERS

Mountain Home

EXPLANATION

Muscovite-biotite granite

Biotite granodiorite (includes

 pegmatite-rich areas)

Hornblende-biotite tonalite

Potassium-rich hornblende-

 biotite granodiorite

Quartz diorite

Kgd

Kt

Na2O < 3.4

Na2O ≤ 3.4 to < 4.0

Na2O ≥ 4.0

B

Kg Kqd

Kgdk

44°

43°30'

43°

116° 115° 114°

Eocene Plutonic Rocks
10 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

Atlanta

Fairfield

Hailey

Idaho City

0 10 20 30 KILOMETERS

Mountain Home

EXPLANATION

Muscovite-biotite granite

Biotite granodiorite (includes

 pegmatite-rich areas)

Hornblende-biotite tonalite

Potassium-rich hornblende-

 biotite granodiorite

Quartz diorite

Kgd

Kt

44°

43°30'

43°

116° 115° 114°

MgO < 0.50

MgO ≤ 0.50 to <1.25

MgO ≥ 1.25

C

Kg

Kgdk

Kqd

Atlanta

Fairfield

Hailey

Idaho City

0 10 20 30 KILOMETERS

Mountain Home

EXPLANATION

Muscovite-biotite granite

Biotite granodiorite (includes

 pegmatite-rich areas)

Hornblende-biotite tonalite

Potassium-rich hornblende-

 biotite granodiorite

Quartz diorite

Kgd

Kt

CaO < 1.5

CaO ≤ 1.5 to < 2.7

CaO ≥ 2.7

D

Kg

Kgdk

Kqd

44°

43°30'

43°

116° 115° 114°
11

Kiilsgaard and others, 1989), thereby resulting in the close spatial relationship. There are, however, alternative geologic processes that could have been involved in the formation of these 

two categories of rocks and these are discussed in a later section 

of this report that discusses the geochemistry of Eocene 

plutonic rocks.

Dioritic Suite

Rocks of the dioritic suite have many common characteristics and are widely exposed across the quadrangle (fig. 2, unit 

Td). The dioritic rocks are darker than the Cretaceous biotite 

granodiorite, and they weather to a reddish, even-granular soil 

that contrasts with the lighter soil derived from most Cretaceous 

plutonic rocks. They invariably contain hornblende and commonly contain augite. The plagioclase is chiefly andesine and is 

strongly and complexly zoned. Quartz tends to be interstitial. 

An unusual amount of magnetite is present, and several stocks 

are the source of magnetic highs on regional aeromagnetic 

maps. Textures of the dioritic rocks vary from equigranular to 

porphyritic, and from fine grained to coarse grained, but most 

rocks are medium grained and seriate. Dramatic variations in 

texture and composition can be present within a single stock, 

and contacts between some phases are gradational in some 

places but sharp in others. The more mafic rocks, particularly 

diorite, are more common at, but not restricted to, margins of the 

stocks. The compositional average of rocks of the suite is quartz 

monzodiorite (fig. 3F).

Gabbro

Several small stocks of gabbro crop out in two areas in the 

Hailey 1°×2° quadrangle. One area is in the central part of the 

quadrangle (unit Tgb of Worl and others, 1991), northwest of 

Fairfield, and the other is in the western part of the quadrangle 

(unit Td of Worl and others, 1991), on Thorn Creek ridge, about 

4 km southwest of Thorn Creek Butte.

The rock on Thorn Creek ridge is dark gray, equigranular, 

and medium to fine grained. It is layered, the layers range from 

millimeters to centimeters in thickness and consist chiefly of 

cumulate aggregates of pyroxene, labradorite grading to 

andesine, and olivine. The plagioclase minerals are strongly 

altered to sericite. The quartz content is low, less than 1 percent. 

Magnetite, zircon, apatite, and sphene are common accessory 

minerals.

The gabbro northwest of Fairfield is rich in hornblende 

(about 50 percent). It is dark colored, equigranular, and medium 

to fine grained. The plagioclase is euhedral andesine that is 

highly altered to sericite and epidote. Relict clinopyroxene, 

potassium feldspar, and opaque minerals are minor constituents. 

Apatite is relatively abundant (about 1 percent).

Diorite

Diorite in the northwestern corner of the Hailey 1°×2°

quadrangle was first recognized by Anderson (1947). Additional exposures of Eocene diorite mapped during this study 

include outcrops south of Wilson Peak, north of Idaho City, and 

southwest of Mt. Heinen (Scanlan, 1986) and a stock on the 

ridge west of Willow Creek in the eastern part of the quadrangle 

(unit Td of Worl and others, 1991). The diorite tends to crop 

out peripheral to or near quartz monzodiorite. It weathers 

readily and commonly is exposed as flat ridge tops or benches. 

Weathered diorite also forms a distinctive, fine-grained, darkbrown soil.

The diorite is dark gray, equigranular to somewhat porphyritic, and medium to fine grained. It commonly has a salt and 

pepper appearance resulting from lighter plagioclase crystals 

interstitial to darker mafic minerals. The plagioclase is andesine 

that is commonly strongly zoned. Hornblende is the principal 

mafic mineral, although biotite, augite, and hypersthene are 

common constituents. Potassium feldspar and quartz are rare. 

Magnetite, zircon, apatite, and sphene are common accessory 

minerals.

Foliated Hornblende-Biotite Granodiorite

Foliated hornblende-biotite granodiorite is well exposed in 

an upthrown fault block north of Fairfield (Lewis, 1990; unit 

Tfgd of Worl and others, 1991). It is resistant to erosion and 

forms steep canyon walls along Little Smoky Creek. The unit is 

heterogeneous, ranging from quartz diorite to granite in composition. The rocks vary from strongly foliated to massive; alternating centimeter- to decimeter-thick layers of mafic and 

subordinate felsic phases define the foliation. Foliation strikes 

northeast and typically dips steeply. The unit is intruded by nonfoliated rocks of the quartz monzodiorite unit described later.

Mafic rocks in the foliated hornblende-biotite granodiorite 

unit are medium to fine grained and equigranular. Plagioclase 

feldspar is euhedral to subhedral oligoclase-andesine. Zoning of 

plagioclase is pronounced; core compositions in one sample are 

about 20-25 percent higher in anarthite content than the rims. 

Hornblende and biotite are the predominant mafic minerals, and 

augite is present locally. Potassium feldspar and quartz are interstitial. Apatite, sphene, allanite, and zircon are present as accessory minerals.

Felsic rocks of the unit are fine-grained, equigranular 

biotite granodiorite and granite, some of which is aplitic. 

Quartz, oligoclase, and perthitic potassium feldspar are the predominant minerals. The mafic minerals are biotite and minor 

hornblende and opaque minerals. Apatite, sphene, allanite, and 

zircon are present in trace amounts.

Quartz Monzodiorite

Quartz monzodiorite ranging to hornblende-biotite granodiorite and hornblende-biotite granite (fig. 3F) is the most widespread Eocene plutonic rock in the Hailey 1°×2° quadrangle 

(unit Tqm of Worl and others, 1991). Anderson (1947) recognized this rock as Tertiary in age and described a belt of quartzhornblende-biotite monzonite porphyry that trends northeast 

from the vicinity of the abandoned town of Quartzburg, which is 

about 4 km northwest of Placerville. Had Anderson used the 

Eocene Plutonic Rocks
12 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

current international classification of granitic rocks (Streckeisen, 

1973), he no doubt would have classified the rock as quartz 

monzodiorite. Other discontinuous exposures of quartz monzodiorite extend from the western edge of the Hailey 1°×2° quadrangle northeast along the broad ridge that separates Grimes 

Creek from Mores Creek to and beyond the northern edge of the 

quadrangle. The largest and most continuous exposures extend 

northeast through Mt. Heinen to beyond Thorn Creek Butte 

(Worl and others, 1991). In this expanse, the rock composition 

undergoes gradational west-to-east change from quartz monzodiorite to hornblende-biotite granodiorite. The persistent northeast trend of the exposures in the western part of the quadrangle 

suggests that emplacement of the quality monzodiorite was 

guided by pre-intrusive northeast-trending faults that are either 

components of, or more or less parallel to, the Trans-Challis 

fault system (Kiilsgaard and Lewis, 1985). Other large exposures of the quartz monzodiorite are in the vicinity of Trinity 

Mountain, and peripheral to the northern and eastern parts of 

Soldier Mountains (Worl and others, 1991).

The quartz monzodiorite typically contains 10-30 percent 

hornblende and biotite. Hornblende is euhedral, acicular, and 

commonly glomerocrystic. Pseudohexagonal books of vitreous 

biotite are a distinctive feature. Clinopyroxene is present in some 

of the quartz monzodiorite both as small phenocrysts and as cores 

of hornblende grains. Euhedral to subhedral plagioclase is 

strongly and complexly zoned and is dominantly andesine. 

Quartz and potassium feldspar (chiefly intermediate microcline) 

are subordinate interstitial minerals. Principal accessory minerals 

are magnetite, apatite, and sphene; minor amounts of allanite, zircon and monazite are also present. Ellipsoidal inclusions of finegrained, hornblende-rich rock are common at some localities.

Biotite Granite

Discontinuous exposures of biotite granite (also called pink 

granite) extend across the Hailey 1°×2° quadrangle (fig. 2, unit 

Tg) (Worl and others, 1991). The largest exposure extends 

northeast, more or less along the Middle Fork of the Boise River, 

from an area about 2 km east of Cottonwood Creek to beyond 

Steel Mountain. The rock in the area was called the Twin 

Springs pluton by Swanberg and Blackwell (1973); however, 

that name is inappropriate because the plutonic rock that crops 

out at Twin Springs, supposedly the type locality, is a small 

stock of hornblende-biotite granodiorite not biotite granite. 

Instead of the name Twin Springs pluton, Lewis and Kiilsgaard 

(1991) proposed that the large, continuously exposed pluton, 

which covers about 470 km2, be named the Sheep Creek 

batholith. The entire drainage area of Sheep Creek, a major tributary of the Middle Fork of the Boise River, is within boundaries 

of the batholith. Other large exposures of biotite granite include 

the Smoky Dome stock, north of Fairfield, and the southern part 

of the Sawtooth batholith, which extends north from the vicinity 

of Atlanta (fig. 2). Similar to exposures of the dioritic suite, 

exposures of the biotite granite show a preferred northeast trend. 

This northeast trend is illustrated by the Sheep Creek batholith 

and smaller stocks of biotite granite to the southwest and by the 

alignment of several stocks that begin southwest of Fairfield and 

extend discontinuously to beyond the northeastern border of the 

quadrangle (Worl and others, 1991).

Coarse euhedral crystals of potassium feldspar account for 

the characteristic pink color of the biotite granite. Finer grained 

varieties of potassium feldspar and some medium-grained varieties are light gray, not pink. These varieties commonly are 

present in small stocks or chilled margins near the contacts of 

larger bodies. Biotite granite typically is equigranular. Modes 

average 30 percent quartz, 35 percent potassium feldspar, and 

35 percent plagioclase (fig. 3E). Subordinate porphyritic variants contain phenocrysts of potassium feldspar. The quartz 

commonly is dark. The potassium feldspar, which is intermediate microcline, is highly perthitic and turbid in thin section. 

Granophyric intergrowths of quartz and potassium feldspar are 

common. The plagioclase is euhedral to anhedral oligoclase 

and commonly has albitic rims. Biotite content averages less 

than 4 percent, and the biotite commonly is partly altered to 

chlorite. Crystals of hornblende may be present, but no primary 

muscovite is present except as rare relict(?) crystals enclosed in 

quartz. Accessory minerals include allanite, apatite, zircon, and 

pyrite. A distinctive characteristic of the biotite granite is the 

presence of small miarolitic cavities, commonly 1-3 mm in long 

dimension, that indicate emplacement at shallow levels. These 

are not present in all exposures but are prominent on high points 

of the Sheep Creek batholith and on peaks of Sawtooth batholith 

northeast of Atlanta.

Petrographic Differences Between Cretaceous and 

Eocene Plutonic Rocks

Petrographic features of Cretaceous and Eocene plutonic 

rocks are described in foregoing paragraphs, but there are diagnostic differences that aid in differentiating the two groups of 

rocks. Large quartz grains, actually clusters of quartz grains 

that have the appearance of a single large grain, are characteristic of Cretaceous plutonic rocks but are not prominent in Eocene 

intrusive rocks. Cretaceous rocks also typically have abundant 

myrmekitic texture and albite-pericline twinning in the potassium feldspar, both of which are rare in Eocene granodiorite. 

The distinct pink color of the Eocene biotite granite is a useful 

field criteria with which to distinguish the rock from most Cretaceous rocks. Granophyric textures and miarolitic cavities are 

also common in Eocene biotite granite, and its highly perthitic 

potassium feldspar is diagnostic. Plagioclase is much more 

strongly zoned in Eocene plutonic rocks. Hornblende is common in Eocene dioritic rocks and in the more mafic Cretaceous 

granodiorite and tonalite but is uncommon in Cretaceous biotite 

granodiorite and two-mica granite, which make up most of the 

Idaho batholith (Lewis and Kiilsgaard, 1991, table 1).

Geochemistry of Eocene Plutonic Rocks

Averages of analytical determinations from 54 samples of 

Eocene plutonic rocks are shown in table 3. As expected, the 

SiO2 content of the rocks decreases from a mean value of 73.8 

percent in biotite granite through progressively lower values in 
13

the quartz monzodiorite, diorite, and gabbro. The K2O content 

also diminishes progressively from granite to gabbro, whereas 

the FeO, MgO, and CaO content increase. Average TiO2 and 

P2O5 contents are highest in diorite. Granite is peraluminous 

(table 3, A/CNK ratios), whereas rocks of the dioritic suite are 

metaluminous.

A distinctive characteristic of the biotite granite is its 

radioactive nature (Swanberg and Blackwell, 1973; Bennett, 

1980a, b; Bennett and Knowles, 1985). Many sediment samples from streams that drain the Sawtooth batholith contain 

anomalous quantities of uranium, as do samples on the unaltered biotite granite (Kiilsgaard and others, 1970). Uranium 

also is present in sediment samples collected from streams that 

drain a stock of biotite granite that crops out near the headwaters of Bear River, along the northern border of the Hailey 

1°×2° quadrangle (Kiilsgaard, 1982). Unpublished National 

Uranium Resource Evaluation (NURE) airborne gamma-ray 

spectrometry surveys of uranium and thorium clearly outline 

exposures of the Sheep Creek batholith, the Smokey Dome 

stock, and smaller stocks in the Boulder Mountains in the 

northeastern corner of the quadrangle.

The biotite granite is also enriched in fluorine, tin, beryllium, molybdenum, and rare-earth elements. Disseminated fluorite is present in biotite granite of the Sawtooth batholith and in 

crosscutting pegmatites and along fractures (Reid, 1963; Pattee 

and others, 1968). Panned-concentrate samples from streams 

draining the Sawtooth batholith are enriched in the above mentioned elements (Kiilsgaard and others, 1970). Rarely did that 

extensive stream-sediment and panned-concentrate sampling 

program find concentrations of the above-mentioned elements in 

samples collected any distance away from the biotite granite. 

Fluorine, beryllium, and molybdenum also are present in sediment samples from streams draining the biotite granite exposure 

at Bear River, in the northern part of the Hailey 1°×2° quadrangle (Kiilsgaard, 1982). Microprobe analyses of biotite from Tertiary granite show that it contains more fluorine than does biotite 

from Cretaceous plutonic rocks (Bennett and Knowles, 1985). 

Tertiary granite in the vicinity of Twin Springs contains anomalous amounts of uranium and molybdenum (Bennett, 1980b). 

Smith (1995) called attention to unusual concentrations of Sn, 

Be, Bi, La, Nb, Th, Y, Mo, Pb, and W in panned-sample concentrates from Devils Creek and Sheep Creek and its tributaries, an 

area in the heart of the Sheep Creek batholith. Some of the 

panned samples have metal concentrations measured in thousands of parts per million. Two hundred and five chip samples 

of biotite granite collected on the ridge between Devils Creek 

and Sheep Creek average 829 ppm F, 24 ppm Sn, and 17 ppm 

Mo, and 105 soil samples from the same locality average 405 

ppm F (Patrick DeWilliams, written commun., 1991). Prospects 

near the junction of Devils Creek and Sheep Creek have been 

explored for beryllium by the Beryllium Corp. of America. 

Molybdenum prospects in the Sheep Creek batholith crop out in 

upper reaches of Roaring River and are reported to contain associated fluorite (Schrader, 1924). Beryl, disseminated in biotite 

granite and as fillings of miarolitic cavities, is present at several 

locations north of North Star Lake and on the ridge between the 

headwaters of Sheep Creek and Smith Creek (Pattee and others, 

1968). The Dismal Swamp placer deposit, near the eastern end 

of the Sheep Creek batholith, contains an assemblage of 

fluorine-bearing and radioactive minerals including, topaz, cyrotolite, xenotime, cassiterite, columbite, monazite, and samarskite (Armstrong, 1957).

The biotite granite has many characteristics similar to those 

of anorogenic granite (Loiselle and Wones, 1979), and these 

characteristics make it different from Tertiary dioritic rocks and 

from granitic rocks of the Cretaceous Idaho batholith (Bennett 

and Knowles, 1985; Lewis and Kiilsgaard, 1991). Anorogenic 

granite magmas have a high fugacity ratio of fluorine to water, 

and the resultant rocks contain associated fluorite (Collins and 

others, 1982; Hannah and Stein, 1990). Manning and Hill 

(1990) noted that, experimentally, residual F-OH-bearing minerals become increasingly enriched in fluorine as melting proceeds. Such enrichment could have resulted in development of a 

dryer magma that crystallized to form biotite granite.

The magmatic origin of the Eocene biotite granite and the 

dioritic suite, however, is debatable. The two categories of rock 

may have crystallized from two separate magmas and not from 

fractional crystallization of a single magma (Lewis, 1984; Stewart, 1987; Lewis and Kiilsgaard (1991). Although the close spatial relationship of the two categories at several different outcrop 

localities suggests a single magmatic source, the spatial relationship could be a structural relationship in which more than one 

magma was intruded along northeast-trending faults. Accepting 

the postulation that F-OH-bearing minerals become increasingly 

enriched in fluorine as melting proceeds (Manning and Hill, 

1990), the dioritic suite could be the product of a first melting 

event, and a second melting event, or a continuation of melting 

of the residual magma, might have resulted in a fluorineenriched and dryer magma that crystallized to form the biotite 

granite. Alternatively, basaltic magma emplaced in the lower 

crust during crustal extension may have been contaminated by 

enclosing rocks as the magma was intruded toward the surface, 

resulting in the formation of the dioritic suite.

Hypabyssal Rocks

Hypabyssal rocks, chiefly dikes and small stocks, are abundant in the Hailey 1°×2° quadrangle. Although little radiometric 

dating has been done on these rocks, their close spatial and 

chemical association with dated Eocene plutons suggests that 

most are of Eocene age. A few mafic dikes may be of Cretaceous age, and basaltic dikes of Miocene age are present locally.

Hypabyssal Rocks of Probable Eocene Age

Countless dikes, swarms of dikes, and small stocks crosscut both Cretaceous and Eocene plutonic rocks in the Hailey and 

Challis 1°×2° quadrangles. The dikes range from andesite to 

rhyolite in composition, are porphyritic, and range in width from 

less than 1 m to more than 100 m and in strike length from a few 

meters to more than 1,000 m.

Dikes of different composition are present throughout the 

region. For purposes of geologic mapping, most of the dikes 

were classed in two suites: a rhyolitic suite of light to 

reddish-tinted felsic dikes and a dacitic suite of dark-green to 

Hypabyssal Rocks
14 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

grayish intermediate to mafic dikes. This field classification 

was adopted because the aphanitic texture and composition of 

the rocks made accurate field identification difficult. 

Identification was further hindered by mineralogical and textural variations within single dikes. Dikes of the rhyolitic 

suite abound near or as crosscutting features in Eocene biotite 

granite, which suggests a genetic relationship. Dikes of the 

dacitic suite, on the other hand, are more numerous near or as 

crosscutting features in Eocene plutonic rocks of the dioritic 

suite, which suggests a genetic relationship with those more 

mafic plutonic rocks.

A significant percentage of the dikes have diverse attitudes, 

but the overwhelming majority strike northeast (Worl and others, 

1991). Emplacement of the dikes probably was guided by the 

northeast-trending pattern of regional faults and associated 

shears that is widespread throughout central Idaho. Fault gouge 

commonly is present along dike walls, particularly in underground mine workings where the rocks are well exposed. Parallel-trending dikes of different compositions commonly are in 

sharp contact with each other, as though a fissure was opened to 

admit one type of dike, only to be reopened to admit another 

type. Locally, dikes are so numerous and so closely spaced to 

each other that they constitute a swarm extending for many kilometers in strike length. At such sites, the dikes may aggregate 

more volume than does the plutonic rock they intrude. Anderson 

(1947) mapped a northeast-trending dike swarm in the northwestern corner of the Hailey 1°×2° quadrangle, a swarm since 

referred to as the Boise Basin dike swarm. Other northeasttrending dike swarms extend along the northwest ridge that 

bounds the southwest-flowing Middle Fork of the Boise River. 

The swarms are particularly conspicuous in the interval between 

Swanholm Creek and Queens River. Still others cross the area 

north of the junction of South Fork of the Boise River and Big 

Smoky Creek and in the area northwest of Camas Prairie (Worl 

and others, 1991).

The northeast-trending regional faults, Tertiary dikes, and 

Eocene stocks and batholiths are principally contained within 

northeast-trending zones of crustal extension that cross the 

Hailey and Challis 1°×2° quadrangles (Kiilsgaard and others, 

1989). In aggregate, the Tertiary dikes, stocks, and batholiths 

constitute an enormous volume of igneous rock that has been 

intruded into the Cretaceous plutonic rocks and other older rocks.

Potassium-argon age determinations of the dikes were not 

attempted. The only exception is a dacite dike sample collected 

northwest of Dollarhide Mountain, which yielded a K/Ar age on 

hornblende of 47.2+0.9 Ma (table 1). The dikes may have been 

intruded over a considerable time span, but dikes that cut Eocene 

plutonic rocks cannot be older than late Eocene. In the vicinity 

of Quartzburg, about 2 km northwest of Placerville, dacitic dikes 

are crosscut by dikes of quartz monzodiorite and by rhyolitic 

dikes. Coarser textured, more porphyritic rhyolite dikes are 

crosscut by finer textured rhyolite dikes.

Dacitic Suite

The dacitic suite consists of chiefly dark colored porphyritic dacite and lesser amounts of highly porphyritic quartz 

monzodiorite and weakly porphyritic andesite.

Dacite dikes (unit Tda of Worl and others, 1991) are dark 

greenish to gray and porphyritic. Phenocrysts range from 2 to 

10 mm in long dimension and make up as much as half of the 

rock mass. Commonly, the rock is bimodal; feldspar phenocrysts of about the same size are set in an aphanitic groundmass 

composed chiefly of feldspar and quartz. Most of the phenocrysts are plagioclase (chiefly zoned andesine), although some 

are quartz that commonly is resorbed and rounded. Small phenocrysts of hornblende and biotite also are common. Magnetite 

is a common accessory mineral. The plagioclase is moderately 

to intensely altered to sericite and the biotite altered to chlorite. 

In the eastern part of the Hailey 1°×2° quadrangle are small 

stocks of dacite porphyry that, except for size, are similar to the 

dacite dikes described above (Worl and others, 1991).

Quartz monzodiorite dikes (unit Tqm of Worl and others, 

1991) have a mineral composition similar to the quartz monzodiorite plutonic rocks but have an aphanitic rather than a granitic 

texture. They differ from the dacite dikes in that they are highly 

porphyritic and contain less than 20 percent quartz. The rock is 

greenish gray; phenocrysts commonly range from 2 to 6 mm in 

size and consist chiefly of zoned andesine and hornblende. The 

groundmass is aphanitic, and the pinkish-gray color results from 

finely crystalline potassium feldspar. Magnetite is a common 

accessory mineral.

Andesite dikes (unit Ta of Worl and others, 1991) are dark 

gray to almost black, fine grained, and weakly porphyritic. 

Many such dikes have been mapped previously as lamprophyre 

(Lindgren, 1900; Anderson, 1939; Anderson and others, 1950). 

Field identification of these dark, fine-grained dikes is so debatable that many dikes designated as andesite may indeed be more 

properly identified as lamprophyre. The dikes weather readily 

and are not seen on soil- or grus-covered surfaces. The phenocrysts are plagioclase (andesine) and commonly are 2-4 mm in 

size. Biotite, various amphibole and pyroxene minerals, and calcic plagioclase are the principal constituents. Laths of andesine 

compose the groundmass in which is interspersed clinopyroxene, hornblende, and magnetite. The silica content is low; an 

average SiO2 value for two samples is 51.8 percent (table 3). 

Distinction between andesite and basalt is difficult, and it is 

likely that some dikes mapped as andesite may be basalt, or vice 

versa.

Some andesite dikes, present locally in Cretaceous plutonic 

rocks, are small, generally only a few centimeters wide but in 

places 2 m or more. They range from a few meters to 100 m or 

so in strike length and have diverse strike attitudes. The dikes 

tend to be of a wispy nature, with one pinching at one locality 

and another one of similar composition starting in an en echelon 

location a short distance away. Some of these mafic dikes may 

Dacite

Quartz Monzodiorite

Andesite
15

be of Cretaceous age, but we have no proof of this age assignment. The assumption is based on location, on the limited number and extent, and on lithologic and structural differences as 

compared to dikes known to be of Tertiary age.

Rhyolitic Suite

Light-reddish to light-gray to tan rhyolite, rhyodacite, and 

quartz trachyte dikes are the principal members of the rhyolitic 

suite.

Rhyolite (unit Tr of Worl and others, 1991) is a common 

Tertiary dike rock in the Hailey 1°×2° quadrangle. It is light 

gray on fresh surfaces but commonly is stained reddish to pinkish by iron oxides on weathered surfaces. It may be weakly to 

intensely porphyritic; phenocrysts of potassium feldspar, plagioclase, and quartz range from 2 to 15 mm in size and are set in an 

aphanitic groundmass of quartz and feldspar. Rounded and 

resorbed phenocrysts of quartz are common, and some have

chalcedonic rims. Spherulitic intergrowths of quartz and potassium feldspar are common in the aphanitic groundmass. The 

plagioclase in phenocrysts and groundmass is oligoclase. Some 

rhyolite dikes contain minor amounts of hornblende. The rhyolite also contains sparse amounts of biotite, which usually is 

altered to chlorite. The feldspar is moderately to intensely 

altered to sericite. Pyrite is a common accessory and, at the outcrop, is typically altered to iron oxides.

Chemically, the rhyolite contains more than 70 percent 

SiO2 (table 3) and is comparable to the biotite granite, with 

which it is probably genetically related. The rhyolite also is 

much more radioactive as measured by a gamma-ray spectrometer than are the other Tertiary dikes, which is another indication 

of its relation to the more radioactive biotite granite.

Rhyodacite (unit Trd of Worl and others, 1991) includes 

dike rock that is pinkish and porphyritic and contains more than 

20 percent quartz and more plagioclase than potassium feldspar. 

The phenocrysts are 2-5 mm in size and are chiefly plagioclase, 

potassium feldspar, and hornblende, with lesser amounts of 

biotite and quartz. The quartz phenocrysts are rounded and 

resorbed. The plagioclase is andesine and is moderately altered 

to sericite. The pinkish groundmass is chiefly fine grained 

potassium feldspar and is characterized by intergrown chalcedonic spherulites.

Large dikes of quartz trachyte (unit Ttr of Worl and others, 

1991) crop out near the head of Blacks Creek, near the southwestern border of the Hailey 1°×2° quadrangle. The dikes form 

spines on the ridges, commonly standing as much as 3 m above

the ground level. Many of the quartz trachyte dikes trend north, 

as opposed to the more common northeast trend of dikes in the 

quadrangle.

Quartz trachyte is light gray to brownish gray and porphyritic and has an aphanitic groundmass. Phenocrysts are dominantly plagioclase (oligoclase-andesine) that has reaction rims 

of albite. They range from 2 to 10 mm in size and are intensely 

altered. The finely crystalline groundmass is dominantly potassium feldspar, which makes up more than 60 percent of the rock. 

Quartz also forms part of the groundmass, but the content is low,

less than 20 percent. Considerable hornblende and some biotite 

also are present but are intensely altered to chlorite. Although 

the trachytic composition is unusual, the dikes are tentatively 

assigned an Eocene age. The presence of hydrous mafic phases 

is characteristic of most Eocene igneous rocks, unlike the pyroxene-bearing rhyolite of the Miocene Idavada Volcanics.

The quartz trachyte dikes differ from rhyolite dikes in that 

they contain less SiO2 and Rb and more Al2O3, iron oxides, 

MgO, CaO, Ba, and Sr (table 3).

Basaltic Dikes of Miocene and Younger(?) Age

The youngest dikes are of diabasic or basaltic composition 

and crosscut all others (unit Tb of Worl and others, 1991). They 

commonly trend north-northwest, across the trend of most other 

Tertiary dikes in the area. Some of the basaltic dikes have been 

dated as Miocene (Armstrong, 1975) and are probably related to 

the Columbia River Basalt Group. Others may be related to the 

Snake River Group basalts and Pleistocene age, in age or even 

younger. The dikes are dark colored and dense, have ophitic to 

subophitic textures, and consist chiefly of microlites of labradorite enclosed by clinopyroxene. Magnetite is common, as are 

sparse amounts of olivine. Quartz content is less than 5 percent. 

The coarser-grained dikes are diabasic, whereas other dikes may 

be andesitic because the plagioclase is andesine rather than 

labradorite.

Summary

Plutonic and hypabyssal rocks underlie much of the Hailey 

1°×2° quadrangle of south-central Idaho. The plutonic rocks are 

of two general ages: Cretaceous rocks of the Atlanta lobe of the 

Idaho batholith and Eocene plutonic and batholithic rocks 

related to the Challis magmatic episode.

The oldest of the Cretaceous plutons consist of pyroxenebiotite quartz diorite, hornblende-biotite tonalite, and potassium-rich hornblende-biotite granodiorite. These rocks are 

intruded by a voluminous mass of biotite granodiorite. Biotite 

granodiorite in the western part of the quadrangle has higher 

contents of SiO2, Na2O, and Ba and lower contents of iron 

oxides, MgO, CaO, TiO2, P2O5, Y, and Zr relative to petrographically similar biotite granodiorite in the eastern part of the quadrangle. The chemical differences probably reflect a differing 

magma source from west to east. In the western half of the quadrangle the biotite granodiorite locally grades into two-mica 

Summary

Rhyolite

Rhyodacite

Quartz Trachyte
16 Plutonic and Hypabyssal Rocks of the Hailey 1°×2° Quadrangle, Idaho

granite, that contains small books of muscovite. Dikes, sills, and 

small stocks of leucocratic granite cut sharply across the other 

Cretaceous units. Pegmatite and aplite dikes are present 

throughout the batholith but are concentrated in a roof zone at 

the southern edge of the Atlanta lobe in the vicinity of Anderson 

Ranch reservoir. Most of the age determinations of Cretaceous 

samples are believed to be too young, because of slow cooling 

and local argon loss. Nevertheless, a review of available data 

indicates that Cretaceous intrusive activity began about 95 Ma 

and continued until about 70-65 Ma.

Eocene plutonic rocks in the Hailey 1°×2° quadrangle can 

be subdivided into two categories: a dioritic suite and biotite 

granite. Rocks of the dioritic suite range in composition from 

pyroxene gabbro to hornblende-biotite granite, and the average 

composition and most common rock type is quartz monzodiorite. The biotite granite is relatively homogeneous and typically 

pink in color. It is enriched in uranium, fluorine, and various 

lithophile elements relative to other plutonic rock types in the 

quadrangle. The two categories of rock may have originated 

from two separate magmas or from a single magma source. The 

Eocene plutons are probably about 50-40 Ma.

Crosscutting all of the plutonic rocks is a myriad of 

hypabyssal dikes, dike swarms, and small stocks. Most of these 

are probably Eocene in age, but few have been dated. Rhyolite 

and dacite are common, but the rocks range from rhyolite to 

basalt in composition. Most, if not all, of the basalts are 

probably Miocene or younger in age.

References

Anderson, A.L., 1939, Geology and ore deposits of the Atlanta District, 

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Table 3. Mean values of major oxides and trace elements, Tertiary plutonic and hypabyssal rocks, Hailey 1°×2° 

quadrangle, Idaho.

[Analytical determinations by X-ray fluorescence and optical spectroscopy. Analysts: J. Taggart, A. Bartel, T. McCollom, C. Papp, P. 

Briggs, E. Brandt, C. Stone, S. Roof, L. Jackson, J. Christie, K. Stewart, K. Slaughter, N. Elsheimer, D. Siems, J. Evans, M. Malcolm, 

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Rock unit Tg Tqm/Tgd Td Tgb Tr Ttr Ta

No. of samples------------ 16 29 5 4 8 3 2

In percent

SiO2 73.8 66.8 54.4 49.2 73.9 64.8 51.8

Al2O3 13.7 15.5 14.3 11.6 13.5 15.6 13.6

Fe2O3 0.72 1.18 4.23 2.24 0.68 2.48 3.61

FeO 0.72 2.03 5.04 7.57 0.86 1.10 5.79

MgO 0.40 1.58 6.51 13.80 0.35 1.06 3.85

CaO 1.25 3.32 8.02 9.65 0.95 1.97 7.36

Na2O 3.68 3.85 3.30 1.83 3.74 3.94 2.75

K2O 4.41 3.35 1.77 0.88 4.30 4.86 1.87

TiO2 0.21 0.49 0.98 0.90 0.19 0.71 1.41

P2O5 0.07 0.19 0.45 0.17 0.03 0.07 0.14

A/CNK 1.04 0.97 0.65 0.54 0.08 1.02 0.68

In parts per million

Ba 820 1370 NA 510 510 1750 1250

Sr 240 610 NA 660 180 620 660

Y 16 13 NA 15 26 21 14

Zr 110 160 NA 90 110 140 170

Rb 160 100 NA 30 170 10 60
17

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